The present invention relates to a method and a device for thermal recording by means of a thermal head having energisable heating elements. Even, more in particular, the invention is related to thermal recording by means of such a thermal head and a radiation beam, even more preferably a transparent thermal head and a laserbeam.
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy. Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
Most of the direct thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
In direct thermal printing, the heating of the thermographic recording material may be originating from image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal print head. The thermal print head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermographic material, e.g. paper, wherein the chemical reaction resulting in colour development takes place. This principle is described in xe2x80x9cHandbook of Imaging Materialsxe2x80x9d (edited by Arthur S. Diamondxe2x80x94Diamond Research Corporationxe2x80x94Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed. 1991, p. 498-499).
A particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. An image can be obtained with such a material because under influence of heat the silver salt is developed to metallic silver.
FIG. 2 (not to scale) shows a cross-section of a composition of a thermographic material m suitable for application within the present invention. The material of the thermographic imaging element 3 comprises a polyethylene terephthalate (PET) support 65 of about 60 to 180 xcexcm (e.g. 175 xcexcm), carrying a subbing layer or substrate 66 of about 0.1 to 1 xcexcm (e.g. 0.2 xcexcml thickness, an emulsion layer 67 of about 7 to 25 xcexcm (e.g. 20 xcexcm) thickness, and a protective layer 68 of about 2 to 6 xcexcm (e.g. 4 xcexcm) thickness (sometimes called top-layer TL). Optionally, on the other side of the PET support 65 a backing layer 69 is provided containing an antistatic and/or a matting agent (or roughening agent, or spacing agent, terms that often are used as synonyms) to prevent sticking. Further details about the composition of such thermographic material m may be read in EP 0 692 733 (in the name of Agfa-Gevaert). The thermographic material can also contain one or more light-to-heat converting agents, preferably in layer 66, 67 or 68. This light-to-heat converting agent is often an infrared absorbing component and maybe added to the thermographic material in any form, e.g. as a solid particle dispersion or a solution of an infrared absorbing dye.
Referring to FIG. 1, there is shown a global principle schema of a thermal printing apparatus 10 that can be used in accordance with the present invention (known from e.g. EP 0 724 964, in the name of Agfa-Gevaert). This apparatus is capable of printing lines of pixels (or picture elements) Pi on a thermographic recording material m, comprising thermal imaging elements or (shortly) imaging elements, often symbolised by the letters Ie. As an imaging element Ie is part of a thermographic recording material m, both are indicated in the present specification by a common reference number 3. The thermographic recording material m comprises on a support a thermosensitive layer, which generally is in the form of a sheet. The imaging element 3 is mounted on a rotatable drum 15, driven by a drive mechanism (not shown) which continuously advances (see arrow Y representing a so-called slow-scan direction) the drum 15 and the imaging element 3 past a stationary thermal print head 16. This head 16 presses the imaging element 3 against the drum 15 and receives the output of the driver circuits (not shown for the sake of greater clarity). The thermal print head 16 normally includes a plurality of heating elements equal in number to the number of pixels in the image data present in a line memory. The imagewise heating of the heating element is performed on a line by line basis, the xe2x80x9clinexe2x80x9d may be horizontal or vertical depending on the configuration of the printer, with the heating resistors geometrically juxtaposed each along another and with gradual construction of the output density. Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element. As the image input data have a higher value, the output energy increases and so the optical density of the hardcopy image 17 on the imaging element 3. On the contrary, lower density image data cause the heating energy to be decreased, giving a lighter picture 17.
In input data block 22, first a digital signal representation is obtained; then, the image signal is applied via a digital interface to a storing means (not shown) of the thermal printer 10.
In the processing unit 24, the digital image signal is processed. Next the recording head 16 is controlled so as to produce in each pixel the density value corresponding with the processed digital image signal value. After processing electrical current may flow through the associated heating elements. In this way a thermal hard-copy 17 of the electrical image data is recorded. By varying the heat applied by each heating element to the carrier, a variable density image pixel is formed.
Although it is known to prepare both black-and-white and coloured half-tone images by the use of a thermal printing head, a need for an improved recording method still exists.
It is an object of the present invention to provide an improved method for recording an image on a thermal imaging element by means of a thermal head having energisable heating elements.
Other objects and advantages of the present invention will become clear from the further description and examples.
The above mentioned object is realised by a method and a system for generating an image on a heat.mode imaging element having the specific features defined respectively in the independent claims and illustrated e.g. in FIG. 3 (to be explained further on). Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.